It is known that the drum machines for wrapping packs of paper napkins comprise an apparatus called "revolver" which consists of a drum rotating about a horizontal axis and provided with a plurality of radical chambers, hereinafter referred to as "revolver pockets", within which the packs of napkins to be wrapped are individually inserted in rapid succession, each of which packs being fed by a driving chain and suitably compressed prior to the introduction thereof into a corresponding revolver pocket. The insertion taking place with the simultaneous interposition of a film of suitable plastic material which is provided for wrapping the napkins, that is, to make up the envelope, after having been properly folded around the sides of the pack and closed by welding and/or glueing. Owing to the high rotary speed of the revolver, it is necessary to feed and compress the napkins at a rate sufficiently high to allow for successively engaging all the revolver pockets, to ensure a convenient productive capacity of the plant. This implies a synchronization, with minimum tolerance margins, of the steps for feeding, squeezing and, then, inserting the napkins into the revolver pockets, mainly as a function of the revolver peripheral speed and of the variation of the momentum to be imparted to each pack prior to the insertion thereof into the revolver pockets. All this calls for the adoption of suitable criteria in the management of the plant, in relation to both the choice of the most suitable operating speeds and the maintenance and adjustment of the individual devices, to allow the complete operative integration and maximum reliability thereof. However, as it is well known, the level and rate of activities for the maintenance and adjustment of the components weigh heavily on the economic efficiency of the plant and, accordingly, on the cost of fabrication of the finished, that is, packaged product. Thus, also in this field of industrial techniques there is a strong demand for adopting simpler constructional criteria to improve the reliability of the production systems and reduce the relevant running costs thereof and, thereby, the cost of fabrication of the packaged product.
Also known, from patent DE 3701273 A, is an apparatus for wrapping packs of paper napkins within envelopes of plastic material, which comprises:
clamp means for compressing the napkins to be packaged which are fed by a driving chain; PA1 means for disposing a predetermined quantity of a film of suitable plastic material in correspondence of each revolver pocket; PA1 a movable arm driven into a reciprocating motion in radial direction with respect to the revolver, to insert each group of compressed napkins into a corresponding pocket and thus carrying out a first wrapping of the film around the napkins; PA1 means for carrying out the welding of the edges of the plastic film wrapped around the napkins, on the front and rear sides thereof, so as to close the envelope; PA1 means for removing the thus wrapped napkins out of the revolver. PA1 progressively compressing, without solution of continuity, the packs to be wrapped, which are fed by a corresponding belt conveyer, and at the same time transferring them towards the revolver of a drum-type wrapping machine at a speed cyclically varying between a minimum value corresponding to the unloading speed of the machine making the packs, and a maximum value at which they are inserted into the revolver pockets; PA1 inserting individually and in continuous succession each of the thus compressed packs into a corresponding pocket of the revolver. PA1 means for progressively compressing, without solution of continuity, the packs to be wrapped which are fed by a belt conveyer and, at the same time, transferring them towards the revolver of a wrapping machine, by means of a pair of superimposed belt conveyers converging towards the revolver and operated by two corresponding driving rollers, the packs being interposed between said belt conveyers individually and in continuous succession; PA1 a speed variator for cyclically varying the feeding speed of said converging belt conveyers, which variator is connected, on one side, to the shaft of a driving member and, on the other, to the shafts of said driving rollers; PA1 means for driving said compressive belts into an alternative rotation of predetermined angular amplitude about the axes of the respective driving rollers.
However, this known apparatus provides for stopping each group of napkins before operating the squeezing and the subsequent insertion thereof into the revolver pockets, and for bringing the compressive means back to the respective initial operating condition before starting the wrapping of a new group of napkins. That is, bringing the compressive means back behind the next group of napkins to be wrapped, so that the feeding thereof to the revolver results discontinuous. Moreover, the functional construction of known apparatus does not allow an increase in the output of the plant, that is, in the napkins-feeding rate, beyond a given value corresponding to the operating speed of the compressive means. In addition to this, there is the fact that the constructional complexity of this known apparatus brings about an intense servicing activity which weighs on the production cost of the packaged product.
Also known form document DE 2322878 is a machine for packaging compressible products such as folded paper napkins and the like, within an envelope of a plastic, able-to-be-wrapped material. The machine comprises a pair of belt conveyers converging towards an articles-compressing device which is made up of a pair of parallel horizontal superimposed belts for moving the napkins delivered from the feeding belts, and also made of a vertical operating cylinder, in a fixed position, provided with a flat head and a corresponding stationary abutment surface, to compress the thus fed napkins. However, this known apparatus also provides for a discontinuous feeding of napkins, owing to the stoppage of the belts of the compressive device, which is necessary to allow for the intervention of the compressive cylinder. Accordingly, the output capacity of the plant cannot be raised beyond a value corresponding to the operating speed of the cylinder.